Tilt device for marine propulsion unit

ABSTRACT

In a tilt device for a marine propulsion unit, a cylinder is constituted by a double pipe having an outer cylinder and an inner cylinder, a rod guide sealing the outer cylinder and the inner cylinder is provided at an end portion of the cylinder, a free piston is arranged in a space between the outer cylinder and the inner cylinder, a liquid discharging chamber is formed in a rod guide side from the free piston of the space, and an absorber valve which can be opened at a time when a pressure of a fluid in the rod side chamber is increased to a level equal to or more than a predetermined value and can feed the fluid in the rod side chamber to the liquid discharging chamber and a return valve which can feed the fluid in the liquid discharging chamber to the rod side chamber are provided in the rod guide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tilt device for a marine propulsionunit.

2. Description of the Related Art

There is a structure described in Japanese Patent No. 2945071 as a tiltdevice with a gas assist function for assisting a tilt up load appliedto a main body of a propulsion unit by a gas pressure in a pressureaccumulating chamber.

The tilt device is structured such that a piston is fixed to the pistonrod inserted to the cylinder. The inner portion of the cylinder isseparated into a rod side chamber and a piston side chamber by thepiston. A switching valve for communicating the rod side chamber withthe piston side chamber and an operating portion thereof are provided ina side portion of the cylinder. A pressure accumulating chamber forapplying a gas pressure for a gas assist to the piston side chamber isarranged within the cylinder and on the piston side chamber.Accordingly, the gas pressure in the pressure accumulating chamber isapplied to both sides of the piston via a working fluid when opening theswitching valve, whereby the gas assist force corresponding to a crosssectional area of the rod can be generated in a tilting up direction.

Further, an absorber valve can be opened when the fluid pressure in therod side chamber is increased to a level equal to or more than apredetermined value in order to absorb an impact force applied to thepropulsion unit main body. When a collision with drifting wood occurs,the fluid in the rod side chamber can be discharged and a return valvecan return the temporarily discharged fluid mentioned above from theabsorber valve to the rod side chamber. This series of events willreturn the propulsion unit main body after rising up. The absorbervalve, rod side chamber and return valve are provided in the piston.Further, a free piston for coinciding the return position of thepropulsion unit main body after rising up with a staying position beforerising up is provided in the piston close to the piston side chamber,whereby it is possible to form a liquid discharging chamber for thefluid discharged from the absorber valve between the piston and the freepiston.

(1) Since the absorber valve and the return valve are provided in thepiston, it is hard to make the diameter of the piston small and further,it is unavoidable that the cylinder is enlarged.

(2) Since the pressure accumulating chamber is within the singlecylinder type cylinder and on the piston side chamber, in a recessedportion in a side of the closed end of the cylinder, it is hard toincrease a capacity of the pressure accumulating chamber.

(3) Since the switching valve and the operating portion thereof areprovided in the side portion of the cylinder, the lateral size of thetilt device becomes too large.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a tilt device for amarine propulsion unit with a gas assist function which can increase thecapacity of the pressure accumulating chamber while being compact andcan secure a rising up performance of a propulsion unit main body causedby a collision of drifting wood.

In accordance with the present invention, the tilt device for a marinepropulsion unit contains a cylinder connected to one of a hull and amain body of a propulsion unit and receiving a working fluid, and apiston rod connected to another and inserted into the cylinder so as tofreely compress and expand. A piston connected to an inserted end to thecylinder of the piston rod and defines a rod side chamber in thereceiving side of a piston rod and a piston side chamber in anon-receiving side of the piston rod within the cylinder. A switchingvalve which is capable of communicating the rod side chamber with thepiston side chamber, and a pressure accumulating chamber capable ofapplying a gas pressure to the piston side chamber are contained in themarine propulsion unit. The cylinder is constituted by a double pipewhich has an outer cylinder and an inner cylinder, and receives thepiston rod and the piston in the inner cylinder. The outer cylinder andthe inner cylinder are sealed at an end portion of the cylinder, a rodguide supporting the piston rod is provided, a free piston is arrangedin a space between the outer cylinder and the inner cylinder, thepressure accumulating chamber is formed in an opposite rod guide side tothe free piston in said space, and a liquid discharging chamber isformed in a rod guide side. An absorber valve which can be opened whenthe fluid pressure in the rod side chamber is increased to a level equalto or more than a predetermined value can feed the fluid in the rod sidechamber to the liquid discharging chamber. A return valve which can beopened when the pressure of the fluid in the liquid discharging chamberis increased to a level equal to or more than a predetermined value canfeed the fluid in the liquid discharging chamber to the rod side chamberare provided in the rod guide.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood from the detaileddescription given below and from the accompanying drawings which shouldnot be taken to be a limitation on the invention, but are forexplanation and understanding only. The drawings

FIG. 1 is a schematic view showing a marine propulsion unit;

FIG. 2 is a cross sectional view of a main portion showing a tilt devicein accordance with a first embodiment;

FIG. 3 is a cross sectional view along a line III—III in FIG. 2;

FIG. 4 is a cross sectional view along a line IV—IV in FIG. 2;

FIG. 5 is a cross sectional view showing a tilt down state;

FIG. 6 is a cross sectional view showing a middle tilt state;

FIG. 7 is a cross sectional view showing a tilt up state; and

FIG. 8 is a schematic view showing a hydraulic circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A marine propulsion unit 10 is, for example, an outboard motor (or aninboard outboard motor) mounted to a hull 11, as shown in FIG. 1, and isstructured such that a clamp bracket 12 is fixed to the hull 11 and aswivel bracket 14 is pivoted to the clamp bracket 12 via a tilt shaft13. A propulsion unit main body 15 of the marine propulsion unit 10 ispivoted to the swivel bracket 14 via a steering shaft (not shown). Anengine unit 16 is mounted on an upper portion of the propulsion unitmain body 15 and a propeller 17 is provided in a lower portion of thepropulsion unit main body 15. The marine propulsion unit 10 is held at adown position by a tilt device 20 mentioned below, and normally orreverse rotates the propeller 17 in accordance with an operation of theengine unit 16 so as to move hull 11 forward or rearward.

The tilt device 20 is structured, as shown in FIGS. 2 to 8, such that amounting portion 26 provided in the cylinder 21 is connected by a pin toone of the clamp brackets 12 and the swivel bracket 14, to the dampbrackets 12 in the present embodiment. A mounting portion 27 is screwedand attached to a piston rod 22 inserted to the cylinder 21 so as tofreely expand and compress is connected by a pin to another of the clampbracket 12 and the swivel bracket 14; to. the swivel bracket 14 in thepresent embodiment. A piston 23 is fixed to an inserted end to thecylinder 21 of the piston rod 22. The cylinder 21 is constituted by adouble pipe (a complex cylinder structure) having an outer cylinder 21Aand an inner cylinder 21B and is structured such that the outer cylinder21A and the inner cylinder 21B are sealed at an end portion of thecylinder 21. A rod guide 24 for supporting the piston rod 22 isprovided. That is, the structure is made such that the inner cylinder21B and the rod guide 24 are held in an inner portion of the outercylinder 21A by abutting one end portion of the inner cylinder 21B to arecess portion in a closed end of the casting outer cylinder 21A.Another end portion of the inner cylinder 21B is fitted to a smalldiameter portion at an inner end of the rod guide 24 fitted to anopening portion of the outer cylinder 21A and caulking and holding therod guide 24 by an outer end portion of the outer cylinder 21A.Reference numeral 24A denotes an oil seal, and reference numerals 24B to24D denote an O-ring. Reference numeral 25 denotes a cap.

The piston rod 22 and the piston 23 are received within the innercylinder 21B of the cylinder 21. The piston 23 defines a rod sidechamber 31 in a side which receives the piston rod 22 and a piston sidechamber 32 in a side which does not receive the piston rod 22 in aninner portion of the inner cylinder 21B. The rod side chamber 31 and thepiston side chamber 32 receive an oil as a working fluid. Referencenumeral 23A denotes an O-ring.

The tilt device 20 has a tilt operating apparatus 40 which makes itpossible to tilt the propulsion unit 15 between a tilt down position anda tilt up position in accordance with a manual operation. The tiltoperating apparatus 40 pierces a communicating passage 41 forcommunicating the piston side chamber 32 with the rod side chamber 31 inthe inserting end side to the inner cylinder 21B of the piston rod 22,and is provided with a switching valve 42 in the communicating passage41. The switching valve 42 is pressed to a valve seat 45 disposed in amiddle portion of the communicating passage 41 by a spring 43B backed upby a spring bearing 43A attached to an opening of the communicatingpassage 41 in the side of the piston side chamber 32, and a springholder 43C so as to close the communicating passage 41. A switchingoperation rod 46 is inserted to a center portion of the piston rod 22from an external portion so as to be installed therein. A front endsmall diameter portion of the switching operation rod 46 abuts theswitching valve 42, and a base end portion of the switching operationrod 46 abuts a cam 48 of an operating portion 47 provided in a bearingportion 28 mounted to the mounting portion 27 attached to a portionprotruding outward from the cylinder 21 of the piston rod 22 (FIG. 3).Reference numeral 29 denotes a cap, reference numeral 28A denotes anO-ring, reference numerals 46A and 47A denote an O-ring, and referencenumeral 49 denotes a connecting pin. The switching operation rod 46 isswitched and set to a closing operation position (an upper end position)of the communicating passage 41 by the switching valve 42 and an openingoperation position (a lower end position) by rotating the cam 48 of theoperating portion 47 by means of a handle engaged and attached to ahandle engaging portion 47B of the operating portion 47, thereby openingand closing the switching valve 42 against an elastic force of thespring 43B.

The tilt device 20 has a pressure accumulating chamber 50 capable ofapplying a gas pressure to the piston side chamber 32, for the purposeof assisting a tilt-up load applied to the propulsion unit main body 15.This occurs when communicating the rod side chamber 31 with the pistonside chamber 32 in accordance with: an opening operation of theswitching valve 42 of the tilt operating apparatus 40 so as to manuallytilt the propulsion unit main body 15. In the present embodiment, anannular space between the outer cylinder 21A and the inner cylinder 21Band a bottom side space of the inner cylinder 21B communicated with theannular space by a communicating passage 51 form a series of pressureaccumulating chambers 50 in the inner portion of the cylinder 21. A freepiston 52 constituting gas-liquid separating means is arranged betweenthe pressure accumulating chamber 50 and the piston side chamber 32 inthe inner portion of the inner cylinder 21B. Reference numeral 52Adenotes an O-ring. The pressure accumulating chamber 50 may be whollyformed as a gas chamber 50A, however, in accordance with the presentembodiment, the structure is made such that an oil chamber 50B isprovided in a lower portion of the gas chamber 50A and the whole of thegas chamber 50A and the oil chamber 50B constitutes the pressureaccumulating chamber 50. The pressure accumulating chamber 50 applies agas pressure generated in the gas chamber 50A when opening the switchingvalve 42 of the tilt operating apparatus 40 to both sides of the piston23 via the working fluid, and reduces the tilt-up load by an assistforce obtained by multiplying a cross sectional area of the piston rod22 by the gas pressure. The cross sectional area corresponds to adifference between both side pressure receiving areas of the piston 23.Further, the pressure accumulating chamber 50 compensates a capacitychange of the rod side chamber 31 and the piston side chamber 32 causedin accordance with forward and rearward movements of the piston rod 22with respect to the rod side chamber 31 and the piston side chamber 32of the cylinder 21, on the basis of expansion and compression of the gaschamber 50A (upward and downward movements of the free piston 52).

The tilt device 20 has the following structure in order to secure arising up performance of the propulsion unit main body 15 due tocollision with an obstacle such as drifting wood or the like. Inaccordance with the present embodiment, the structure is made such thata free piston 61 is arranged in an annular space between the outercylinder 21A and the inner cylinder 21B of the cylinder 21. An oppositeside of the rod guide 24 with respect to the free piston 61 in theannular space forms the pressure accumulating chamber 50, and a side ofthe rod guide 24 forms a liquid discharging chamber 60. Referencenumerals 61A and 61B denote an O-ring. Further, an absorber valve 62 (aspring bearing 62A, a spring 62B and a spring holder 62C) and a returnvalve 63 are provided in the rod guide 24 (FIG. 4). The absorber valve62 can be opened when the oil in the rod side chamber 31 is increased toa level equal to or more than a predetermined value such as underapplication of an impact force generated by collision with an obstaclesuch as drifting wood or the like. The absorber valve 62 can feed theoil in the rod side chamber 31 to the liquid discharging chamber 60, andcan extend the piston rod 22 and the piston 23 so as to raise thepropulsion unit main body 15 up. The return valve 63 can be opened whenthe oil in the liquid discharging chamber 60 is increased to a levelequal to or more than a predetermined value via the piston side chamber32 and the pressure accumulating chamber 50 under application of anempty weight of the propulsion unit main body 15 tilted up afterabsorbing the impact force generated by the collision with the obstacle.Oil in the liquid discharging chamber 60 can return to the rod sidechamber 31, and can compress the piston rod 22 and the piston 23 so asto return the propulsion unit main body 15 to a position before risingup. The free piston 61 is brought into contact with the end surface ofthe rod guide 24 so as to make the capacity of the liquid dischargingchamber 60 zero before absorbing the impact force. The free piston 61moves apart from the end surface of the rod guide 24 at a degreecorresponding to the capacity of the oil which the absorber valve 62feeds from the rod side chamber 31 for absorbing the impact force (thecapacity of the liquid discharging chamber 60 becomes the capacity ofthe oil). The free piston 61 returns all of the oil from the returnvalve 63 to the rod side chamber 31 after absorbing the impact force soas to be brought into contact with the end surface of the rod guide 24.Further, the free piston 61 returns the piston rod 22 and the piston 23to the position before absorbing the impact force and coincides thereturn position of the propulsion unit main body 15 with the stayingposition before jumping up.

In this case, the tilt device 20 is provided with an operating valve 64(a spring bearing 64A, a spring 64B and a spring holder 64C) in thepiston 23. The operating valve 64 is opened when the oil pressure in thepiston side chamber 32 is increased to a level equal to or more than apredetermined value such as a case that a forward propelling force equalto or more than a predetermined value is applied under a condition ofshipping over a shoal in which the propulsion unit main body 15 is keptat an optional middle tilt position. Accordingly, the piston rod 22moves forward to the cylinder 21 so as to be compressed, and thepropulsion unit main body 15 is downed to a down position and becomes anormal shipping state.

A description will be given below of contents of the operation of thetilt device 20.

(A) Normal Shipping

A description will be given of a reverse lock state keeping the tiltdevice 20 at a tilt down position shown in FIG. 5 at a normal shippingtime. In this case, the operating portion 47 of the tilt operatingapparatus 40 sets the switching valve 42 to a closed position, and thepropulsion unit main body 15 abuts the front end portion of the swivelbracket 14 to the forward movement keeping portion of the clamp bracket12, whereby the propulsion unit main body 15 is kept at the normalshipping position.

In the case of setting the propulsion unit main body 15 to the normalshipping position and shipping forward, the forward movement propellingforce is supported by the forward movement keeping portion of the clampbracket 12, and the tilt device 20 is held at the tilt down position. Onthe other hand, at a rearward shipping time, a tensile force is appliedto the piston rod 22 due to the rearward movement propelling force andthe pressure of the rod side chamber 31 is increased, however, theabsorber valve 62 is not opened by the pressure increased of thisdegree, and the tilt device 20 is held at the tilt down position.

(B) Shipping over Shoal

In shipping over the shoal, it is necessary to change the tilt device 20from a tilt down position shown in FIG. 5 to a middle tilt positionshown in FIG. 6 so as to prevent the propulsion unit main body 15 fromhitting the bottom of the sea or river. In this case, the switchingvalve 42 is opened by the operating portion 47 of the tilt operatingapparatus 40 in the manner mentioned above so as to communicate the rodside chamber 31 with the piston side chamber 32. Accordingly, the pistonrod 22 and the piston 23 freely move in a vertical direction withrespect to the cylinder 21. Therefore, it is possible to manually setthe propulsion unit main body 15 to a desired middle tilt position. Atthis time, the gas pressure in the pressure accumulating chamber 50assists the tilt-up load as mentioned above, whereby the tilt-upoperation can be easily performed.

The piston rod 22 and the piston 23 are in an unmovable tilt lock statewith respect to the cylinder 21 by closing the switching valve 42 inaccordance with the operating portion 47 of the tilt operating apparatus40 in a state of setting the propulsion unit main body 15 to the middletilt position so as to make the rod side chamber 31 not communicatedwith the piston side chamber 32. Accordingly, it is possible to shipover the shoal. In this case, at a time of landing the hull, it ispossible to set the propulsion unit main body 15 to the middle tiltposition in the same manner as mentioned above.

In this case, in order to again position the propulsion unit main body15 at the tilt-down position, the switching valve 42 is again opened bythe operating portion 47 of the tilt operating apparatus 40 so that thepropulsion unit main body 15 slowly returns to the tilt-down positiondue to the empty weight.

(C) Collision of Drifting Wood at Rearward Shipping over Shoal

If an obstacle such as drifting wood or the like collides with thepropulsion unit main body 15 and an impact force directed towards afront portion of the hull is applied to the lower portion of thepropulsion unit main body 15 at a time of rearward shipping over theshoal in a state of setting the tilt device 20 to the middle tiltposition shown in FIG. 6, the compressing force is applied to the pistonrod 22, and the pressure of the oil in the piston side chamber 32 isincreased. When the oil pressure reaches a level equal to or more than apredetermined value, the operating valve 64 provided in the piston 23 isopened so as to absorb the impact, the piston rod 22 enters into thecylinder 21 so as to be compressed, and the propulsion unit main body 15is downed in a tilt-down direction.

(D) Change from Shipping over Shoal to Normal Shipping

In the case of setting the tilt device 20 to the middle tilt positionshown in FIG. 6 so as to ship over the shoal and thereafter shipoffshore and change the propulsion unit main body 15 to the tilt-downposition to perform a normal shipping, the forward propelling force ofthe propulsion unit main body 15 is increased. The propulsion unit mainbody 15 is applied a force forward the hull due to the propelling force,so that a compressing force is applied to the piston rod 22 and thepressure of the oil in the piston side chamber 32 is increased. When theoil pressure reaches a level equal to or more than a predeterminedvalue, the operating valve 64 provided in the piston 23 is opened, thepiston rod 22 enters into the cylinder 21 so as to be compressed, andthe propulsion unit main body 15 is positioned at the tilt-downposition.

(E) Collision of Drifting Wood at Forward Shipping

If an obstacle such as drifting wood or the like collides with thepropulsion unit main body 15 and an impact force directed to a rearportion of the hull is applied to the lower portion of the propulsionunit main body 15 when shipping forward with the tilt device 20 set tothe tilt-down position shown in FIG. 5 or the middle tilt position shownin FIG. 6, the tensile force is applied to the piston rod 22, and thepressure of the oil in the rod side chamber 31 is increased. When theoil pressure reaches a level equal to or more than a predeterminedvalue, the absorber valve 62 provided in the rod guide 24 is opened soas to absorb the impact force, the piston rod 22 moves out of thecylinder 21 so as to be expanded, the tilt device 20 is upped to thetilt-up position shown in FIG. 7, and the propulsion unit main body 15is jumped up. At this time, the oil which the absorber valve 62discharges from the rod side chamber 31 is fed to the liquid dischargingchamber 60, and the free piston 61 moves apart from the end surface ofthe rod guide 24 at a degree corresponding to the capacity of the oil.

After the propulsion unit main body 15 is jumped up, the pressure of thepiston side chamber 32 is increased due to the empty weight of thetilted-up propulsion unit main body 15, and the pressure increases thepressure in the liquid discharging chamber 60 via the pressureaccumulating chamber 50. When the pressure in the liquid dischargingchamber 60 is increased to a level equal to or more than a predeterminedvalue, the return valve 63 is opened so as to return the oil in theliquid pressure chamber 60 to the rod side chamber 31. Accordingly, whenthe free piston 61 is brought into contact with the end surface of therod guide 24, the absorber valve 62 returns all of the oils fed to theliquid discharging chamber 60 due to the impact absorption to the rodside chamber 31, so that the tilt device 20 returns to the positionbefore absorbing the impact, and the propulsion unit main body 15returns to the position before jumping up.

In accordance with the present embodiment, the following effects can beobtained.

(1) Since the pressure accumulating chamber 50 is formed in the spacebetween the outer cylinder 21A and the inner cylinder 21B in thecylinder 21, it is possible to form the pressure accumulating chamber 50with a large capacity in a wide range in a longitudinal direction of thecylinder 21, and the capacity of the pressure accumulating chamber 50can be increased while the tilt device 20 is compact, whereby a gasassist performance can be improved.

(2) Since the free piston 52 corresponding to the liquid-gas separatingmeans is interposed between the pressure accumulating chamber 50 and thepiston side chamber 32, the gas in the pressure accumulating chamber 50does not feed to the piston side chamber 32 and further to the rod sidechamber 31 through the switching valve 42 even when the tilt device 20takes any mounting attitude such as a normal or reverse position.Accordingly, the pressure accumulating chamber 50 can maintain asufficient gas capacity, no gas is mixed to the working fluid in thepiston side chamber 32 and the rod side chamber 31, and it is possibleto secure a stable gas assist function at a time of opening theswitching valve 42 and secure a stable tilt-lock function at a time ofclosing the switching valve 42. That is, it is possible to secure animproved operability while keeping a freedom in the mounting direction.

(3) The absorber valve 62 and the return valve 63 are in the rod guide24 and have a diameter larger than that of the piston 23 in the complexcylinder type cylinder 21. Accordingly, since it is possible to receivethe absorber valve 62 and the return valve 63 in the rod guide 24 withroom without increasing the diameter of the rod guide 24 and it ispossible to make the diameter of the piston 23 small, it is possible tojump up the propulsion unit main body 15 against the collision ofdrifting wood while making the cylinder 21 small-diameter and compact.

(4) In connection with the matter that the absorber valve 62 and thereturn valve 63 are provided in the rod guide 24, the free piston 61 forcoinciding the returning position after jumping up of the propulsionunit main body 15 with the staying position before rising up is providedin the space between the outer cylinder 21A and the inner cylinder 21Bin the cylinder 21. The liquid discharging chamber 60 for the fluiddischarged from the absorber valve 62 is formed in the rod guide 24 sidefrom the free piston 61 in the space. Accordingly, it is possible tocoincide the returning position of the propulsion unit main body 15jumped up due to the collision of drifting wood with the stayingposition before rising up.

(5) Since the switching valve 42 is provided in the piston rod 22 or thepiston 23 and the operating portion 47 of the switching valve 42 isprovided in the portion protruding from the cylinder 21 of the pistonrod 22, it is possible to reduce a lateral size of the tilt device 20and make the tilt device 20 compact.

(6) Since the absorber valve 62 and the return valve 63 in the items (3)and (4) are not provided in the piston rod 22 or the piston 23 but inthe rod guide 24 at a time of providing the switching valve 42 in thepiston rod 22, it is possible to easily place the switching valve 42without increasing the diameter of the piston 23.

As heretofore explained, embodiments of the present invention have beendescribed in detail with reference to the drawings. However, thespecific configurations of the present invention are not limited to theembodiments but those having a modification of the design within therange of the present invention are also included in the presentinvention.

As mentioned above, in accordance with the present invention, the tiltdevice for a marine propulsion unit provided with a gas assist functioncan increase the capacity of a pressure accumulating chamber while beingcompact and can secure a jump-up performance of a propulsion unit mainbody caused by a collision with drifting wood.

Although the invention has been illustrated and described with respectto several exemplary embodiments thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions and additions may be made to the present invention withoutdeparting from the spirit and scope thereof. Therefore, the presentinvention should not be understood as limited to the specific embodimentset out above, but should be understood to include all possibleembodiments which can be embodied within a scope encompassed andequivalents thereof with respect to the features set out in the appendedclaims.

What is claimed is:
 1. A tilt device for a marine propulsion unitcomprising: a cylinder connected to one of a hull and a main body of apropulsion unit which is arranged and constructed to receive a workingfluid; a piston rod being connected to the other of the hull and themain body of the propulsion unit and being insertable into the cylinderso as to freely compress and expand; a piston connected to an insertedend of the piston rod, and defining a rod side chamber in a side whichdoes not receive a piston rod within the cylinder; a switching valvecapable of communicating the rod side chamber with the piston sidechamber; and a pressure accumulating chamber capable of applying a gaspressure to the piston side chamber, wherein the cylinder has a doublepipe having an outer cylinder and an inner cylinder, the piston rod andthe piston are received in the inner cylinder, a rod guide supportingthe piston rod seals the outer cylinder and the inner cylinder at an endportion of the cylinder, a free piston is arranged in a space betweenthe outer cylinder and the inner cylinder, the pressure accumulatingchamber is formed in a opposite rod guide side to the free piston insaid space, a liquid discharging chamber is formed in a rod guide side,and an absorber valve which can be opened at a time when a pressure of afluid in the rod side chamber is increased to a level equal to or morethan a predetermined value and can feed the fluid in the rod sidechamber to the liquid discharging chamber and a return valve which canbe opened at a time when the pressure of the fluid in the liquiddischarging chamber is increased to a level equal to or more than apredetermined value and can feed the fluid in the liquid dischargingchamber to the rod side chamber are provided in the rod guide.
 2. A tiltdevice for a marine propulsion unit as claimed in claim 1, wherein saidswitching valve is provided in the piston rod or the piston, a switchingoperation rod operating said switching valve is installed in the pistonrod, and an operating portion of said switching operation rod isprovided in a portion protruding from the cylinder of the piston rod. 3.A tilt device for a marine propulsion unit as claimed in claim 1,wherein a gas-liquid separating means is arranged between said pressureaccumulating chamber and the piston side chamber.
 4. A tilt device for amarine propulsion unit as claimed in claim 2, wherein a gas-liquidseparating means is arranged between said pressure accumulating chamberand the piston side chamber.
 5. A tilt device for a marine propulsionunit as claimed in claim 3, wherein said gas-liquid separating means isa free piston.
 6. A tilt device for a marine propulsion unit as claimedin claim 4, wherein said gas-liquid separating means is a free piston.7. A tilt device for a marine propulsion unit as claimed in claim 1,wherein a communicating passage communicating the piston side chamberwith the rod side chamber is provided in an inserted end side to theinner cylinder of said piston rod, and said switching valve is providedin the communicating passage.
 8. A tilt device for a marine propulsionunit as claimed in claim 2, wherein a communicating passagecommunicating the piston side chamber with the rod side chamber isprovided in an inserted end side to the inner cylinder of said pistonrod, and said switching valve is provided in the communicating passage.